Method for starting a steam generator comprising a heating gas channel that can be traversed in an approximately horizontal heating gas direction and a steam generator

ABSTRACT

The invention relates to a steam generator comprising a heating gas channel, which can be traversed in an approximately horizontal heating gas direction and in which at least one continuous heating surface is located, configured from a number of approximately vertical evaporator tubes, connected in parallel to allow the passage of a flow medium. The aim of the invention is to provide a method for starting a generator, which guarantees a high degree of operational safety, even for a steam generator with a particularly simple construction. According to the invention, to achieve this, at least several evaporator tubes are partially filled to a predeterminable desired level with unevaporated flow medium, prior to the impingement of the heating gas channel by a heating gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP02/09312, filed Aug. 20, 2002 and claims the benefit thereof.The International Application claims the benefits of Europeanapplication No. 01121027.5 EP filed Aug. 31, 2001, both of theapplications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a method for starting a steam generator with aheating gas channel that can be traversed in an almost horizontalheating gas direction, in which at least one continuous heating surfaceis located, configured from a number of approximately verticalevaporator tubes, connected in parallel to allow the passage of a flowmedium. It also refers to a steam generator of this kind.

BACKGROUND OF INVENTION

In a gas and steam turbine system, the heat contained in the expandedoperating medium or heating gas from the gas turbine is used to generatesteam for the steam turbine. The heat transfer takes place in awaste-heat steam generator downstream of the gas turbine, in whichusually a number of heating surfaces are arranged for water preheating,steam generation and steam superheating. The heating surfaces areconnected to the water-steam circuit of the steam turbine. Thewater-steam circuit normally contains several, e.g. three, pressurestages, each having an evaporator heating surface.

Several alternative design concepts are possible for the steam generatorconnected downstream of the heating gas end of the gas turbine as awaste-heat steam generator, i.e. as a once-through steam generator or asa circulating-steam generator. With a once-through steam generator, theheating of the steam generating tubes provided as evaporator tubescauses evaporation of the flow medium in the steam generator tubes in asingle passage. In contrast to this, with a natural- orforced-circulation steam generator the circulated water is onlypartially evaporated during one passage through the evaporator tubes.The water not evaporated is re-supplied to the same evaporator tubes forfurther evaporation after removal of the generated steam.

A once-through steam generator is, in contrast to a natural- orforced-circulation steam generator, not subject to pressure limitationand therefore live steam pressures far above the critical pressure ofwater of (P_(Kri)≈221 bar) where there are still only slight differencesin density between the liquid-similar and steam-similar medium, arepossible. A high live steam pressure favors a high thermal efficiencyand therefore low CO₂ emissions in a power station heated by fossilfuel. In addition, a once-through steam generator is of simplerconstruction than a circulating-steam generator and therefore can bemanufactured at particularly low cost. The use of a steam generatordesigned on the once-through principle as a waste heat steam generatorof a gas and steam turbine system is therefore particularly suitable forachieving a high overall efficiency of the gas and steam turbine systemcombined with simple construction.

Particular advantages with regard to the cost of manufacture and alsothe necessary maintenance work is offered by a waste-heat steamgenerator of horizontal design where the heating medium or heating gas,in particular the waste gas from the gas turbine, is passed through thesteam generator in an almost horizontal direction of flow. A horizontalsteam generator of this kind is known from EP 0 944 801 B 1. Because ofits design as a once-through steam generator, the overflowing of waterfrom the evaporator tubes forming the continuous heating surface intothe downstream superheater must be prevented during operation. However,this can cause problems, particularly when starting the steam generator.

When the steam generator is started, a water discharge, as it is called,can occur. This arises when the flow medium in the evaporator tubesinitially evaporates due to the heating of the evaporator tubes andthis, for example, takes place in the middle of the particularevaporator tube. This causes the quantity of water downstream (alsoknown as water plugs) to be expelled from the particular evaporatortube. To reliably make sure that unevaporated flow medium from theevaporator tubes cannot reach the superheater connected after the tubes,the known steam generator, such as normally also a once-through steamgenerator of vertical design, is provided with a water-steam separatoror precipitator connected between the evaporator tubes forming thecontinuous heating surface and the superheater. Surplus water is drawnoff from this and either returned to the evaporator by a circulatingpump or discharged. A water-steam separating system of this kind is,however, comparatively expensive with regard to both design andmaintenance.

SUMMARY OF INVENTION

The object of the invention is therefore to provide a method forstarting a steam generator of the type stated above, that alsoguarantees a high degree of operating safety combined with aparticularly simple construction. Furthermore, a steam generatorparticular suitable for the performance of the method is also to bespecified.

With regard to the method, this object is achieved according to theinvention in that at least several of the evaporator tubes forming thecontinuous heating surface are partially filled to a predeterminabledesired level with unevaporated flow medium before heating gas isapplied to the heating gas channel.

The invention is based on the consideration that to maintain a highoperational safety when the steam generator is starting, the entry ofunevaporated flow medium into the superheater connected after theevaporator tubes must be safely prevented. For a particularly simpleconstruction, however, this should be ensured without the water-steamseparating device normally provided with once-through steam generators.To achieve this, with a steam generator of horizontal construction wherean outlet connector connected to the outlet end of the evaporator tubesforming the continuous heating surface is directly connected to an inletdistributor of the superheater, only partial filling of the evaporatortubes with unevaporated flow medium should take place before starting.The amount, and therefore the desired level, for this initial fillingprior to the impingement of the heating gas channel by a heating gasshould be chosen so that on the one hand the water discharge due to theinitial steam formation is avoided and on the other hand inadequatecooling of the evaporator tubes during starting is precluded.

The desired level is appropriate chosen so that at the beginning of thestarting operation the supply of the evaporator tubes with flow mediumcan be omitted. Thus, during the starting process, i.e. after theheating gas channel has been impinged by the heating gas, evaporation ofthe flow medium already present in the evaporator tubes first takesplace. In this process the unevaporated flow medium, within theparticular evaporator tube downstream of the particular location of thestart of evaporation, is shifted by the forming steam bubble into thezone of the particular evaporator tube that previously was unfilled.There, this amount of the unevaporated flow medium can evaporate or, ifsufficiently low mass flow densities are maintained in the evaporatortubes, again drops into the lower space of the particular evaporatortube. By choosing a suitable desired level, the part area of theparticular evaporator tube located in the upper area of the particularevaporator tube, initially not filled with flow medium and serving as acompensating space for the column underneath as a flow medium, can thusbe designed to be large enough to securely prevent the outflow ofunevaporated flow medium from the particular evaporator tube, even atthe start of evaporation.

During the partial filling of the particular evaporator tubes before theinitial impingement of the heating channel by heating gas, the actuallevel of the particular evaporator tubes is advantageously matched tothe predeterminable desired level. To do this, the actual full level isadvantageously determined by measuring the pressure difference betweenthe lower tube inlet and upper tube outlet of the particular evaporatortube, with the value obtained in this way being appropriately used as abasis for the supply of the particular evaporator tube with unevaporatedflow medium.

Depending on the operating state of the steam generator and its previoushistory, different time characteristics of the heating of the steamgenerator during its starting phase can be provided. To guarantee aparticularly reliable maintenance of the boundary conditions even whenthe pattern of the starting phase varies that both reliably precludesthe outflow of unevaporated flow medium from the evaporator tubes and ineach case guarantees adequate cooling of all evaporator tubes, thedesired level for the initial filling of the evaporator tubes isadvantageously determined on the basis of the design heatingcharacteristics on starting. The characteristics of the heating onstarting are determined appropriately using characteristic values forthe boiler geometry and/or time characteristics of the heat supply bythe heating gas. For this purpose, characteristics of heating onstarting matched to a number of parameter combinations of this kind canbe stored in a database assigned to the steam generator, with it beingpossible in particular to take account of the heating cycles precedingthe current heating cycle.

In the starting phase of the starting process, i.e. in a time spanimmediately after the start of impingement of the heating gas channelwith heating gas, operation of the steam generator is provided withoutfurther impingement of the evaporator tubes by flow medium or feedwater. However, it is appropriate for the supply of feed water orunevaporated flow medium to the evaporator tubes to take place after theonset of steam formation in the evaporator tubes, so that after steamformation has occurred an adequate cooling of the particular evaporatortube is ensured in each case. The onset of steam formation in this caseis advantageously detected by a pressure rise in the water-steamcircuit. To enable the evaporator tubes to be supplied with feed waterto meet demand in a particularly reliable manner, a measured valuecharacteristic of the pressure of the flow medium is thus advantageouslymonitored after the start of impingement of the heating gas channel byheating gas, whereby if this measured value then exceeds a predeterminedlimit, a continuous impingement of the evaporator tubes by feed watertakes place.

After the start of supply of the feed water to the evaporator tubes, thefeed water supply to the evaporator tubes is controlled so that anoutflow of unevaporated flow medium from the evaporator tubes issecurely avoided. To do this, the supply of feed water to the evaporatortubes is advantageously controlled in such a way that superheated steamemerges from the upper tube outlet of that or every evaporator tube. Toguarantee that no evaporated flow medium can reach the downstreamsuperheater, the provision at the outlet of the evaporator tubes ofsteam that is comparatively only weakly superheated is sufficient.

To guarantee a particularly high operating stability of the steamgenerator, the mass flow density of the flow medium being fed to theevaporator tubes is set in such a way that an evaporator tube heatedmore than a different evaporator tube with the same continuous heatingsurface has a higher throughput of flow medium than the other evaporatortube. The continuous heating surface of the steam generator thus, withregard to the flow characteristics of a natural circulation evaporatorheater (natural circulation characteristics), has, if different heatingpatterns of individual evaporator tubes occur, a stabilizing behaviorthat leads to a matching of the outlet temperature, without the need ofan external influence even with differently heated evaporator tubesconnected in parallel at the flow medium end. To guarantee thischaracteristic, an impingement of the evaporator tubes by acomparatively low mass flow density is provided.

This object is achieved with regard to the steam generator in that acommon differential pressure measuring device is allocated to adistributor connected to the inlet of the evaporator tubes and to anoutlet collector connected to the outlet of the evaporator tubes. Thedifferential pressure measuring device enables the level in theevaporator tubes to be monitored in a particularly satisfactory manner,so that a characteristic value for this can be used as a suitable guidevalue for the supply of the evaporator tubes.

The particular advantages of the invention are that just by a partialfilling of the evaporator tubes with the unevaporated flow medium beforean initial impingement of the heating gas channel by heating gas astarting process with high operating safety is guaranteed, thus,particularly with adequate cooling of the evaporator tubes, theadmittance of unevaporated flow medium to the superheater downstream ofthe evaporator is securely avoided, therefore enabling a particularlysimple construction of the steam generator. In this case thecomparatively expensive water-steam separating system can be completelydone away with whilst maintaining the high operating safety standard,without the need to use particularly robust or high-quality rawmaterials in the construction in its place. A particularly secure andstable operating behavior is thus especially achievable in that theevaporator tubes are impinged at comparatively low mass flow density, sothat unevaporated flow medium in the evaporator tubes also remains inthe particular evaporator tube even at the onset of steam formation inthe particular evaporator tube and is finally also evaporated there.

BRIEF DESCRIPTION OF THE DRAWING

An example of the embodiment of the invention is further explained withthe aid of a drawing. The drawing is a simplified representation showinga longitudinal section of a steam generator of horizontal construction.

DETAILED DESCRIPTION OF INVENTION

The steam generator 1 in accordance with the illustration is connectedto the outlet gas end of a gas turbine (not illustrated in more detail)as a waste-heat steam generator. The steam generator 1 has a surroundingwall 2 that forms an almost horizontal heating gas channel 6 for theexhaust gas from the gas turbine, that can be traversed in the heatinggas direction x shown by the arrow 4. The heating gas channel 6 containsa number of evaporator heating surfaces designed according to thecontinuous principle, also designated a continuous heating surface 8,10. The exemplary embodiment shows two continuous heating surfaces 8,10, but also just one or a greater number of continuous heating surfacescan be provided.

The continuous heating surfaces 8, 10 of the steam generator 1 eachconsist of a number of parallel evaporator tubes 14 or 15 in the form ofa tube bundle to allow the passage of a flow medium W. The evaporatortubes 14, 15 are in this case each aligned almost vertically, with anumber of evaporator tubes 14 or 15 being arranged side-by-side viewedin the heating gas direction x. In each case, only one of the evaporatortubes 14 or 15 arranged side-by-side in this way is visible.

A common distributor 16 is connected before the evaporator tubes 14 ofthe first continuous heating surface 8 at the flow medium end and acommon outlet collector 18 is connected to the outlet end. The outlet ofthe outlet collector 18 of the first continuous heating surface 8 isconnected via a drop pipe system 20 to a distributor 22 allocated to thesecond continuous heating surface 10. The outlet of the secondcontinuous heating surface 10 is connected to an outlet collector 24.

The evaporator system formed by the continuous heating surfaces 8, 10can be impinged by the flow medium W that is evaporated by a singlepassage through the evaporator system and is drawn off from the outletof the evaporator system as steam D and fed to a superheater surface 26connected to the outlet collector 24 of the second continuous heatingsurface 10. The pipe system formed by the continuous heating surfaces 8,10 and the superheater surface 26 connected after them is connected tothe water-steam circuit of a gas turbine (not illustrated in moredetail). In addition, a number of other heating surfaces 28, in eachcase schematically indicated, are connected to the water-steam circuitof the gas turbine. The heating surfaces 28 can, for example, bemedium-pressure evaporators, low-pressure evaporators and/or preheaters.

The evaporator system formed by the continuous surfaces 8, 10 isdesigned in such a way as to be suitable for a supply of the evaporatortubes 14, 15 at a comparatively low mass flow density, with theevaporator tubes 14, 15 having a natural circulation characteristic.With this natural circulation characteristic, an evaporator tube 14 or15 heated more than a different evaporator tube 14 or 15 with the samecontinuous heating surface 8 or 10 has a higher throughput of flowmedium W than the other evaporator tube 14 or 15.

The illustrated steam generator 1 is of comparably simple construction,In this case, the main difference is that the second continuous heatingsurface 10 is connected directly to the superheating surface 26connected after it, omitting a comparatively expensive water-steamseparating system or precipitation system, so that the outlet collector24 of the second continuous heating surface 10 is directly connected viaan overflow line, and without other components connected in between, toa distributor of the superheating surface 26. However, to also maintaina comparatively high operating safety with this comparatively simpledesign in all operating states, the steam generator 1 is operated withinthese boundary conditions when starting. In this case, the steamgenerator 1 is particularly operated on starting in such a way that onthe one hand there is sufficient cooling of the evaporator tubes 14, 15forming the continuous heating surfaces 8, 10 and also of the steamgenerator tubes forming the superheating surface 26. On the other hand,the steam generator 1 is also operated in such a way on startup thatalso without a water-steam separating system connected between thesecond continuous heating surface 10 and the superheating surface 26,the supply of unevaporated flow medium W to the superheating surface 26is securely avoided.

To guarantee this, the evaporator tubes 14 forming the first continuousheating surface 8 are filled to a predeterminable desired level, shownby the dotted line 30 in the illustration, with unevaporated flow mediumW, before the initial impingement of the heating gas channel 6 withheating gas from the upstream gas turbine. The filling of the evaporatortubes 14 with unevaporated flow medium W before the commencement ofheating in this case takes place through the feed water line and thedistributor 16 that are present in any case. In this way, the actuallevel achieved in the evaporator tubes 14 is determined by measuring thepressure difference between the lower distributor 16 and the upperoutlet collector 18. For this purpose, a common differential pressuremeasuring device 32 is allocated to the distribution 16 and outletcollector 18. Using the actual level in each evaporator tube 14,determined in this way, further filling with unevaporated flow medium Wis controlled so that the predetermined desired level is obtained withina predetermined tolerance band.

On completion of the initial filling of the evaporator tubes 14 withunevaporated flow medium W, further supply of the flow medium W to theevaporator tubes 14 is halted. In this condition, the beginning of theactual starting process for the steam generator 1 takes place, with, inparticular, the impingement of the heating gas channel 6 with heatinggas from the upstream gas turbine taking place. Due to the heating ofthe evaporator tubes 14 that has now begun, the unevaporated flow mediumW in the evaporator tubes begins to evaporate. A local evaporation thentakes place in each of the evaporator tubes 14 after a certain timeperiod, with the still unevaporated flow medium W downstream or abovethe actual location of the start of evaporation being shifted to theupper zone of the particular evaporator tube 14 initially not filledwith flow medium W. There, an evaporation of this part of the flowmedium W takes place, or this part of the flow medium W drops back intoits lower area due to the comparatively low design mass flow density inthe evaporator tubes 14.

Any unevaporated flow medium W still remaining is passed through thedrop pipe system 20 into the next downstream second continuous heatingsurface 10 and there it is completely evaporated. The second continuousheating surface 10 thus takes the still remaining water discharge fromthe first continuous heating surface 8 in each case. Because theevaporator tubes 14 are only partly filled before the start of theactual starting process, no, or practically no, unevaporated flow mediumW enters the outlet collector 24 connected after the second continuousheating surface 10 or the superheater surface 26 connected after theoutlet collector.

The exemplary embodiment thus provides for only partial filling of theevaporator tubes 14 forming the first continuous heating surface 8; thesecond continuous heating surface 10 initially remains unfilled.Additionally, in an alternative form of embodiment, evaporator tubes 15forming the second continuous heating surface 10 can also be partiallyfilled using a similar method.

A determination of whether steam production has already started in theevaporator tubes 14 and evaporator flow medium or steam D has enteredthe outlet collector 24 is determined by measuring the pressure of theflow medium W or steam D, particularly at the outlet collector 24 or theoutlet of the superheating surface 26. A measured value characteristicof the pressure of the evaporated flow medium or steam D in the outletcollector 24 or at the outlet of the superheating surface is detectedand monitored by means of a suitable arranged pressure sensor. Thisenables the start of steam production to be inferred on the basis of anincrease in pressure, which can reach several bars per minute when steambegins to form.

After the onset of steam formation in the evaporator tubes 14 has beendetected in this way, the operating supply of feed water or unevaporatedflow medium W to the distributor 16 allocated to the continuous heatingsurface 8 takes place. During the further starting process, i.e.particularly until a steady-state operating condition is reached, thesupply of feed water or unevaporated flow medium W to the evaporatortubes 14 is controlled in such a way that superheated steam D, i.e.steam D without a wet component, emerges at the upper tube outlet 34 ofthe evaporator tubes 14.

Moreover, the mass flow density of the flow medium W being supplied tothe evaporator tubes 14 is set so that an evaporator tube 14 heated morethan a different evaporator tube 14 has a higher throughput of flowmedium W than the other evaporator tube 14. This ensures that thecontinuous heating surface 8 has a self stabilizing behavior in themanner of the flow characteristics of a natural-circulation evaporatorheating surface even if different heating of individual evaporator tubes14 occurs.

The performance, shown here, of the starting process of the steamgenerator 1 ensures adequate cooling for the evaporator tubes 14, 15 atall times and also that no unevaporated flow medium W enters thesuperheating surface 26 connected after the second continuous heatingsurface 10 at any time. Compliance with these boundary conditions inthis case is to be particularly ensured by the choice of the desiredlevel for the evaporator tubes 14 before beginning the actual startingprocess. The desired level for the evaporator tube 14 is predeterminedso that precisely these boundary conditions are complied with as a basisfor the design starting process. To do this, the desired level is presetfor steam generator 1 depending on the design heating characteristics onstarting. The heating characteristics on starting in this case aredetermined from characteristic values for the boiler geometry andmaterial and/or the type of fuel. In particular, it can be provided inthis case that a number of possible starting heating characteristicssuitable for the steam generator 1 in question are stored in a memorymodule as a type of database, from which a characteristic matched to theactual situation can be selected using operating data and used as abasis for specifying the desired level.

1. A method for starting a steam generator, comprising: traversing a heating gas channel with a heating gas in an approximately horizontal heating gas direction; locating at least one continuous heating surface, configured from a number of approximately vertical evaporator tubes connected in parallel to allow the passage of a flow medium; partially filling at least several evaporator tubes to a predeterminable desired level with an unevaporated flow medium; and impinging the heating gas channel by the heating gas, wherein during a supply of the evaporator tubes with the flow medium, a particular mass flow density is set so that an evaporator tube heated more than a different evaporator tube of the same continuous heating surface has a higher throughput of flow medium than the different evaporator tube.
 2. The method in accordance with claim 1, wherein the level of the unevaporated flow medium in the particular evaporator tubes is determined by a differential pressure measurement between the lower tube inlet and upper tube outlet.
 3. The method in accordance with claim 2, wherein the desired level is specified relative to design starting heating characteristics.
 4. The method in accordance with claim 3, wherein the starting heating characteristics are determined on the basis of characteristic values for the boiler geometry and/or the time characteristics of the heat supply by the heating gas.
 5. The method in accordance with claim 4, wherein a measured value characteristic of a pressure of the flow medium is measured when the heating gas channel is impinged by heating gas, whereby if the measured value exceeds a predeterminable limit, a continuous impingement of the evaporator tubes by unevaporated flow medium takes place.
 6. The method in accordance with claim 5, wherein the supply of flow medium to the evaporator tubes takes place after the onset of steam formation in the evaporator tubes.
 7. The method in accordance with claim 6, wherein the supply of flow medium to the evaporator tubes is controlled in such a way that superheated steam emerges at the upper tube outlet of the one or every evaporator tube. 